A number of systems exist which use an existing radio receiver (e.g., an AM and/or FM tuner in a vehicle) for playback of audio signals from an auxiliary source such as a compact disc (CD) player or a tape cassette player. For example, as disclosed in U.S. Pat. No. 4,712,250, to Michels et al, an adapter is provided to connect a portable stereo cassette player to a vehicle radio receiver. The adapter comprises a standard audio plug for receiving a signal from the cassette player. The adapter comprises circuitry (e.g., a tuning capacitor) for modulating the signal onto a weak FM carrier on an unused FM channel. The adapter can be inserted into a cigarette lighter receptacle to couple the FM signal to the radio.
Another system is disclosed in U.S. Pat. No. 5,448,757, to Hirata, in which the output of a CD player is FM-modulated for reception by an FM tuner. A radio frequency for modulating the output of the CD player is selected manually.
The devices disclosed in the two aforementioned patents are both disadvantageous for the same reason, that is, these devices require manual steps to play the audio signals from an auxiliary source on a radio receiver. A need exists for a device which allows an existing radio to automatically play an audio signal from an auxiliary source, instead of standard AM or FM broadcast programming, whenever the auxiliary source is present and operational.
In U.S. Pat. No. 3,751,601, to Wally, a tape playback device is provided for playing an audio signal from a tape cassette through a radio receiver in lieu of FM broadcast programming. A switch is provided for automatically switching between a radio broadcast station and a tape cassette source. The switch is mechanically activated when a tape is inserted into the tape playback device. This device, however, does not directly detect the presence of a signal from the auxiliary source. A need therefore exists for a device which detects when an audio signal from auxiliary source is present to provide the audio signal through a tuner. A need also exists for such a device which does not require activation by a user such as manual tuning or insertion of a tape or cassette into a playback device.